This invention relates generally to oxide thin films and composites including such materials as can be derived from suitable metal precursors and/or prepared using molecular beam epitaxy techniques. In particular, the present invention relates to use and/or integration of such oxide films in a variety of devices.
In recent years there has been increasing interest in thin oxide films as alternative high xcexa dielectrics. For example, one such material is MgO, which has a dielectric constant of 9.4. Because of its chemical stability and thermodynamic compatibility with silicon (Si), interfacial reactions should be minimal. To date, a variety of deposition techniques have been used to grow MgO thin films on Si including metal-organic chemical vapor deposition (MOCVD), B. S. Kwak, E. P. Boyd, K. Zhang, A. Erbil and B. Wilkins, Appl. Phys. Left., 54 25(1989); E. Fujii, A. Tomozawa, S. Fujii, H. Torii, M. Hattori and R. Takayama, Jpn. J. Appl Phys., 32, L1448(1993); sputtering, S. Fujii, A. Tomozawa, E. Fujii, H. Torii, R. Takayawa and T. Hirao, Appl. Phys. Lett., 65(11), 1463(1994); S. Kim and S. Hishita, Thin Solid Films, 281-282, 449(1996); M. Tonouchi, Y. Sakaguchi and T. Kobayashi, J. Appl. Phys., 62(3), 961(1987); Y. Li, G. C. Xiong, G. J. Lian, J. Li and Z. Gan, Thin Solid Films, 223,11(1993), laser deposition, T. Ishiguro, Y. Hiroshima and T. Inoue, Jpn. J. Appl. Phys., 35, 3537(1996); P. Tiwari, S. Sharan and J. Narayan, J. Appl. Phys., 69(12), 8358(1991); D. K. Fork, F. A. Ponce, J. C. Tramontana and T. H. Geballe, Appl. Phys. Left., 58(20), 2294(1991); S. Amirhaghi, A. Archer, B. Taguiang, R. McMinn, P. Barnes, S. Tarling and I. W. Boyd, Appl. Surf. Sci., 54, 205(1992); e-beam-assisted molecular, beam epitaxy (MBE) F. J. Walker, R. A. McKee, S. J. Pennycook and T. G. Thnndat, Mater. Res. Soc. Symp. Proc., 401, 13(1996); and the sol-gel method J. G. Yoon and K. Kim, Appl. Phys. Lett., 66(20), 2661(1995) However, as would be understood by those skilled in the art, deposition techniques of the prior art are accompanied by various shortcomings and/or deficiencies with respect to the use and applications described herein.
For instance, much current work has focused on the formation of dielectrics by chemical vapor deposition. Because such deposition techniques operate at pressures in excess of 10xe2x88x923 Torr, oxidation of a silicon substrate prior to thin film deposition provides an unwanted formation of silicon dioxide. Other work has been performed using electron-beam evaporation of MgO onto Si in an oxygen ambient. The inherent mechanical difficulties associated with using electron-beam sources in an oxygen atmosphere has limited this approach, especially for large area coverage. The technique also requires atomistic control of the amount and type of atomic species deposited to enable epitaxy. This complication has further limited the success of this approach.
A related concern is the integration of functional components on substrates, for instance integration of a ferroelectric thin film on a semi-conductor substrate. It is often difficult to deposit/grow epitaxial thin films of the sort needed for optimal performance on such substrates because of interactive diffusion between the layers, oxidation of the substrate and similar such material or surface incompatibilities. Capping components, typically oxides, can be layered on the substrate to alleviate such difficulties for instance to act as a diffusion barrier. However, introduction of such components can present also promote substrate oxidation and the non-epitaxial growth of a component subsequently integrated thereon.
Given the problems and deficiencies in the art relating to the preparation and use of oxide thin films, there is a need for an improved approach to such films and composites so as to maximize the benefits available therefrom; e.g., for use thereof with integrated devices.
Accordingly, it is an object of the present invention to provide various deposition methods and techniques overcoming the problems and shortcomings of the prior art, including those described above, such that the resulting films can be used in conjunction with integrated systems and devices. It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of this invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of this invention.
It can be an object of this invention to provide a thin film fabrication technique which combines the versatility of standard chemical vapor deposition with the molecular beam properties of conventional molecular beam epitaxy.
It can be another object of this invention to provide, in particular, for the use of metal organic molecular beam epitaxy (MOMBE) with suitable metal-organic precursors, such precursors having, without limitation: 1) a very low background pressure ( less than 10xe2x88x927 Torr); 2) a single metal-containing volatile species in the molecular beam, 3) a high sticking coefficient, 4) and high long-term stability at the evaporation temperature (no decomposition or oxidation of the source materials), 5) an organic component/ligand accessible commercially or by known synthetic techniques, and 6) sufficient volatility ( greater than 10xe2x88x926 Torr) at low temperature.
It can also be an object of the:present invention to provide a method for deposition of thin films of nanometer dimensions (less than 50 nm, and preferably less than about 3 to about 10 nm) by molecular beam epitaxy techniques using pressures (oxygen and/or base) lower than those of one prior art and/or to achieve the film composition and/or morphology desired.
It can also be an object of the present invention to provide methods, processes, and/or techniques of the type described herein for the deposition of a variety of alkaline earth and rare earth oxides, from commercially-available metal precursors, without carbon contamination on substrates useful in the fabrication of integrated systems.
It can also be an object of the present invention to provide thin film metal oxides, the metal component thereof including but not limited to Ca, Zn, Cd, V, Rh, Pt, Th, Fe, Pd, Ga, Al, Be, U, Hf, La, Pr, Sm, Gd, Dy, Er, Yb, Sc, Th, Ti, Ba, Ce, Mg and Pb, such films as can be amorphous or provided with glass, crystalline, polycrystalline or epitaxial morphologies.
It can also be an object of the present invention to provide metal oxide films, such as those referenced above, for use in the fabrication of a variety of composites for subsequent integration, such composites including substrates of Si and Ge, or with compound (Group III-V and II-VI) substrates.
It can also be an object of this invention to provide a deposition technique which allows in situ film diagnosis and monitoring, allowing for use of reflection high-energy election diffraction and other diagnostic methods (including MS and RGA) under the vacuum conditions inherent to such a technique.
It can also be an object of the present invention to integrate metal oxide thin films, such as those described above, and/or related composites thereof to provide for one or more devices, such devices having, without limitation, dielectric, refractory, ferro-electric, electro-optical, non-linear optical, field emission, phosphor/luminophor, optical isolation, photonic, waveguide, MOS, MOSFET, semi-conducting, and/or super conducting applications, such devices as can otherwise be fabricated using techniques and components known to those skilled in the art.
In particular, it can also be an object of the invention to provide epitaxial MgO layers on silicon substrates have potential applications in ultra-large scale integration and in microphotonics. Because of the thermodynamic stability of MgO with both silicon and ferroelectric oxide thin films, it can be used as an ideal buffer layer between ferroelectric oxides and Si in optical integrated circuits. Furthermore since its dielectric constant of 9.65 is 3 times larger than SiO2, crystalline thin film MgO is a potential alternative gate dielectric to SiO2 in metal-oxide-semiconductor (MOS) devices and in metal/ferroelectric/insulator/semiconductor field effect transistor devices.
Other objects, features, benefits and advantages of the present invention will be apparent from the following summary, descriptions and examples, and will be readily apparent to those skilled in the art having knowledge of various thin film materials, composites and/or integrated devices. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying examples, tables, data, incorporated references and all reasonable inferences to be drawn therefrom.
In part, this invention relates to the deposition of thin films by MOMBE using a suitable oxide precursor in a plasma O2 atmosphere. The deposited films have been characterized by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and capacitance-voltage measurements. As described and illustrated below, films with bulk-like dielectric properties are deposited. This technique utilizes solid metal-organic compounds as the metal source. See, J. P. Bade, E. A. Baker, A. I. Kingon, R. F. Davis, and I. Bachmann, J. Vac. Sci. Technol. B, 2, 327(1990); A. L. Kingon, K. Y. Hsieh, L. L. H. King, S. H. Rou, K. J. Bachmann and F. Davis, Mat. Res. Sco. Symp. Proc., 200, 49(1990); D. J. Lichtenwalner, and A. I. Kingon, Appl. Phys. Lett. 59(23), 3045(1991); S. Ikegawa, and Y. Motoi, Thin Solid Films, 281-282, 60(1996); K. Hayama, T. Togun, and M. Ishida, J. Cryst. Growth, 179, 433(1997); and European Patent Appln. 0055459A, publd. Jul. 22, 1997.
Accordingly, the present invention includes a method for deposition of a metal oxide layer on a substrate. The inventive method includes (1) providing a suitable substrate; (2) providing a metal-organic precursor compound; and (3) depositing a film of the metal oxide on the substrate by molecular beam epitaxy deposition. The metal oxide deposited on the substrate is derived from the precursor compound utilized in conjunction with molecular beam epitaxy techniques and equipment of the sort described herein and as would be otherwise well known to those skilled in the art made aware of this invention. The inventive methodology of this invention provides for metal oxide deposition substantially without substrate oxidation.
A variety of oxidants/oxygen sources can be utilized, as would be well known to those skilled in the art. Preferred embodiments of this invention utilize oxygen plasma generated at appropriate energy levels to provide suitable oxygen partial pressures. In preferred embodiments, the method of this invention includes such partial pressures less than about 10xe2x88x923 Torr. As described elsewhere herein, depending upon choice of substrate and other system parameters suitable oxygen partial pressures can be less than about 10xe2x88x925-10xe2x88x926 Torr. Regardless, the growth of oxide thin films is observed without substrate oxidation.
With regard to the aforementioned metal-organic precursor compounds, the metal component thereof can include, but is not limited to, Ca, Zn, Cd, V, Rh, Pt, Th, Fe, Pd, Ga, Al, Be, U, Hf, La, Pr, Sm, Gd, Dy, Er, Yb, Sc, Th, Ti, Ba, Ce, Mg and Pb. The ligands used in conjunction with such metal components are as described herein or as can provide the precursor compound sufficient volatility and/or decomposition properties for use in such deposition techniques. Various such ligands can include those derived from straight-forward modifications and extensions of the work described herein, and/or through routine experimentation by those skilled in the art made aware of this invention. The precursor compounds of this invention can have, under deposition conditions described, a partial pressure of less than about 10xe2x88x923-10xe2x88x924 Torr.
Regardless, preferred embodiments of the present methodology include such precursor compounds which provide for the deposition of refractory metal oxides and/or reactive metal oxides, depending upon end use application and specific properties required of the resulting film. Such films can be amorphous or provided with glass, crystalline, polycrystalline and/or epitaxial morphologies. In preferred embodiments, such films are crystalline and stoichiometric, such as the magnesium oxide thin film characterized herein.
As described more fully below, control of substrate temperature can be part of the inventive methodology. Generally, such a substrate can be heated to a temperature of less than about 1000xc2x0 C. For many composites and/or end use applications, a preferred substrate is silicon, which can be heated to a temperature of between about 500xc2x0 C. and 1000xc2x0 C. Other such substrates can comprise other Group IV elements, various transition metals and semi-conductor materials. Compound substrates are also contemplated in conjunction with this invention. Regardless, an appropriate substrate temperature can be determined and/or adjusted with consideration of various other factors described herein, including but not limited to substrate identity and/or oxygen partial pressure.
In preferred embodiments, the present method also includes formation of a carbide layer on the substrate, prior to deposition of a metal oxide film. Typically, a metal-organic precursor compound, of the type described above, is likewise a precursor for the carbide layer, such formation the result of substrate exposure to the precursor absent an. oxygen source. Such carbide layers are preferably formed with a silicon substrate, but can also be formed in conjunction with other substrate materials, including transition metals and semi-conductors. In such embodiments, the carbide layer can be epitaxial with respect to the substrate, and as would be desired for subsequent deposition of an epitaxial metal oxide. Even so, non-epitaxial carbide layers can be formed and utilized as needed, e.g.xe2x80x94for substrate oxidation resistance.
While MOMBE has been used to deposit compound semi-conductor layers, it has not been utilized to any significant degree for the deposition/growth of oxide films. A major advantage of MOMBE methods described herein, is that the O2 pressure is very low ( less than 10xe2x88x926 Torr) compared to prior art techniques (xcx9c10xe2x88x923 Torr), which can enable the growth of oxide thin films directly on Si without formation of an amorphous SiO2 interfacial layer, a condition which inhibits epitaxial coverage. Furthermore, the technique can be scaled to large area substrates. The high vacuum also enables in situ surface diagnosis such as reflection high-energy electron diffraction (RHEED), a diagnosis which is not possible with MOCVD and other techniques of the prior art. This invention can be extended to include deposition and use of a wide variety of refractory and reactive metal oxides, using available metal-organic precursors.
Thin dielectric films, including but not limited to MgO, deposited on Si and such substrates are of considerable technological importance. For example they can be used as an optical isolation layer for ferroelectric oxide waveguide on Si. See, U.S. Pat. No. 5,776,621, in particular FIG. 1 and cols. 3-8 thereof, the entirety of which is incorporated herein by reference. They can also be used as a gate oxide in MOS devices; R. A. Mckee, F. J. Walker and M. F. Chisholm, Phys. Rev. Letts., 81(14), 3014(1998) or they can serve as a buffer between other oxides and Si wafer where the integration of these materials with such a substrate is required; K. J. Hubbard and D. G. Schlom, J. Mater. Res., 11(11), 2757(1996).
Accordingly, the present invention also includes a composite comprising a substrate and a metal oxide film thereon. The composite is substantially absent substrate oxidation, and the oxide film is substantially without carbon contamination. The film is produced by metal-organic molecular beam epitaxy deposition, such deposition conducted under oxygen partial pressures less than about 10xe2x88x923 Torr. The substrate temperature can be as required to preclude oxidation thereof, with consideration of partial oxygen pressure. However, at pressures of the type described herein, the substrate temperature is typically less than about 1000xc2x0 C.
The film component of such a composite can comprise the oxide of a metal of the type described above, or as could otherwise be provided by a corresponding metal-organic precursor compound. In preferred embodiments and depending upon end use application, the metal oxide is selected from a group consisting of refractory metal oxides and reactive metal oxides. As described herein, thin films of such refractory metal oxides can exhibit bulk dielectric properties.
The film component of the present composites can have, as described above, various morphologies. In preferred embodiments, such a film is epitaxial, and can be utilized for subsequent deposition of other functional materials, such as various super conducting or semi-conducting materials. As such, the composites of this invention can further include a carbide layer between the substrate and the metal oxide film. While the carbide layer can be non-epitaxial, epitaxy is preferred with respect to subsequent oxide deposition, phase-pure crystallinity, bulk dielectric properties and/or use of such composites/oxides in conjunction with various integrated devices. As described elsewhere herein, the carbide interlayer can be the carbonization product of the substrate and a suitable metal-organic precursor compound, such carbonization resulting from treatment of a precursor compound under molecular beam epitaxy conditions absent the presence of oxygen.
The use of epitaxial, uniform functional oxide thin films on various substrates, such films and substrates including those described herein, can also be used in conjunction with deposition of high crystalline quality, epitaxial high-temperature-superconducting (HTSC) oxides. For instance, an HTSC oxide film can subsequently be epitaxially deposited on an insulated substrate structure using any of the existing growth techniques currently used for single crystal MgO substrates. As a result, wafers of Si coated with MgO can be marketed to researchers growing such ferroelectric or superconducting films, as a replacement for single crystal oxide substrates.
Other integrated elements, composites and/or devices can be prepared in accordance with this invention using fabrication techniques otherwise known in the art and modified as provided herein, such elements, composites and devices including but not limited to those described in U.S. Pat. Nos. 5,418,216, 5,830,270 (in particular, FIGS. 5 and 6 and the descriptions thereof), U.S. Pat. Nos. 5,482,003 and 5,323,023xe2x80x94all of which are incorporated herein by reference in their entirety.